BACKGROUND: Three important results came from the Amsterdam Growth and Health Longitudinal Study (AGAHLS). This study followed three birth cohorts (1962, 1963 and 1964) of boys and girls in the Amsterdam region in the Netherlands. The follow-up period was 25 years, with 10 measurements from age 12 to 42 years. The main purpose of the AGAHLS was to detect changes in health and lifestyle over time during the teenage and young adult period. METHODS: In total, 617 subjects were recruited from two secondary schools in Amsterdam and Purmerend. We measured aerobic fitness (VO2 peak), bone mineral density (BMD), obesity from body mass index (BMI) and body fatness from the sum of four skinfolds (S4S). Daily physical activity (DPA) was measured from heart rate, pedometers and an interview. Daily food intake (DFI) was measured by a cross-check dietary history interview. RESULTS: Longitudinal data analyses revealed that: (1) aerobic fitness, as measured by direct measurement of maximal oxygen uptake (VO2 max), increased more significantly in the physically active percentile (P > 75) of males and females than in the physically inactive percentile (P < 25), (2) BMD, as measured with dual X-ray absorptiometry (DEXA) in the wrist, hip and lumbar region, showed that physical activity in youth with a high mechanical load on the bones (mostly weight bearing) increased bone formation in the hip and lumbar region of males and females in adulthood, (3) the longitudinal relationship between DPA and DFI with the development of overweight and obesity (measured from BMI and S4S) showed that more DPA resulted in significantly lower fat mass, but no relationship could be demonstrated with DFI . CONCLUSIONS: The main conclusion from this 25-year longitudinal research is that the promotion of physical activity (including physical education and sport) in adolescence can potentially be a strong tool to prevent chronic diseases and reduce healthcare costs later in life.